Conventionally, techniques such as an “oil hydraulic hybrid” technique and an “oil hydraulic servo” technique are known. There are oil hydraulic hybrid techniques roughly classified into two types as described in Non-Patent Document 1. One of the types is, in place of a conventional oil hydraulic servo system having low efficiency, a hybrid oil hydraulic system that drives a conventional oil hydraulic pump with an inverter drive motor or a servo motor to make it possible to perform valve control without generating wasteful energy. This system is popularly prevalent in the industrial circles.
The other is of a type in which excessive mechanical energy is regenerated into a battery through an electric motor mainly and that is mainly used in an automobile or a construction machine. Such a type is also called a hybrid type. In particular, since hybrid automobiles are explosively popularized in the automotive industry, in general, it is strongly recognized that a hybrid means complex use of petroleum and an electric motor. However, as described in Non-Patent Document 2, an oil hydraulic hybrid automobile is researched or developed overseas. This means a technique that uses an oil hydraulic motor and an accumulator in place of an electric motor and a battery, respectively, to accumulate mechanical (fluid) energy obtained in a braking state or the like. The object of the technique is just energy regeneration. The technique is different from a technique used in the present invention (will be described later).
As a technique related to the present invention, an oil hydraulic servo system is given (this servo system mentioned here means a system to automatically track target values such as a position, a speed, and a power). The hydraulic servo systems, as described in Non-Patent Document 3, can be classified into a conventional valve control type having a constant pressure and a constant discharge rate and a relatively recently developed pump control type. A popularly used inexpensive oil hydraulic drive circuit is configured by an open circuit that generates pressured oil with a main pump, restricts the pressured oil with a valve to drive an actuator, and returns the pressured oil to a tank. A typical example of a valve control type servo system is a system that uses a high-performance proportional valve and a servo valve to improve the responsiveness and precision of an actuator. A typical example of a pump control type servo system is a system that is improved in efficiency by performing load sensing drive of a variable displacement pump or controlling the rotating speed of a fixed displacement pump with an inverter motor or a servo motor. As described in Non-Patent Document 4, an oil hydraulic drive circuit in which two or more pumps are serially coupled to each other to obtain a pressure-increasing effect is also given.